Information handling system with immediate scheduling of load operations in a dual-bank cache with single dispatch into write/read data flow

ABSTRACT

An information handling system (IHS) includes a processor with a cache memory system. The processor includes a processor core with an L1 cache memory that couples to an L2 cache memory. The processor includes an arbitration mechanism that controls load and store requests to the L2 cache memory. The arbitration mechanism includes control logic that enables a load request to interrupt a store request that the L2 cache memory is currently servicing. The L2 cache memory includes dual data banks so that one bank may perform a load operation while the other bank performs a store operation. The cache system provides a single dispatch point into the data flow to the dual cache banks of the L2 cache memory.

This invention was made with United States Government support underAgreement No. HR0011-07-9-0002 awarded by DARPA. The Government hascertain rights in the invention.

BACKGROUND

The disclosures herein relate generally to information handling systems(IHSs), and more specifically, to cache memory systems that IHSs employ.

Information handling system (IHSs) employ processors that processinformation or data. Current day processors frequently include one ormore processor cores on a common integrated circuit (IC) die. Aprocessor IC may also include one or more high-speed cache memories tomatch a processor core to a system memory that typically operates atsignificantly slower speeds than a processor core and the cache memory.The cache memory may be on the same integrated circuit (IC) chip as theprocessor or may be external to a processor IC. Processor corestypically include a load-store unit (LSU) that handles load and storerequests for that processor core. Before accessing system memory, theprocessor attempts to satisfy a load request from the contents of thecache memory. In other words, before accessing system memory in responseto a load or store request, the processor first consults the cachememory.

BRIEF SUMMARY

In one embodiment, a processor memory caching method is disclosed. Themethod includes sending, by a processor element, a plurality of requestsfor memory operations to a cache memory, the cache memory includingfirst and second cache banks, the operations including load operationsand store operations, each load and store operation exhibiting arespective size requirement. The method also includes arbitrating, by anarbitration mechanism, among the plurality of requests for memoryoperations to select a particular load operation and a particular storeoperation for access to the cache memory. The method further includescommencing, by the arbitration mechanism, the particular load operationon the first cache bank during a first cache cycle. The method stillfurther includes commencing, by the arbitration mechanism, theparticular store operation on the second cache bank during a secondcache cycle that follows the first cache cycle. The method also includesperforming, by the first and second cache banks, the particular loadoperation and the particular store operation substantiallysimultaneously.

In another embodiment, a cache memory system is disclosed. The cachememory system includes a processor element. The cache memory system alsoincludes a cache memory, coupled to the processor element, that receivesa plurality of requests for memory operations. The cache memory includesfirst and second cache banks. The operations include load operations andstore operations, each load and store operation exhibiting a respectivesize requirement. The cache memory includes an arbitration mechanismthat arbitrates among the plurality of requests for memory operations toselect a particular load operation and a particular store operation foraccess to the cache memory. The arbitration mechanism commences theparticular load operation on the first cache bank during a first cachecycle and commences the particular store operation on the second cachebank during a second cache cycle that follows the first cache cycle. Thefirst and second cache banks perform the particular load operation andthe particular store operation substantially simultaneously.

In another embodiment, an information handling system (IHS) is disclosedthat includes the cache memory system described above coupled to asystem memory.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings illustrate only exemplary embodiments of theinvention and therefore do not limit its scope because the inventiveconcepts lend themselves to other equally effective embodiments.

FIG. 1 shows a block diagram of one embodiment of the disclosedinformation handling system (IHS).

FIG. 2 shows a block diagram of a processor integrated circuit thatincludes the disclosed cache management system.

FIG. 3A is a data flow diagram of a chiplet that includes the disclosedcache management system that employs a single-bank cache memory.

FIG. 3B is a data flow diagram of a chiplet that includes the disclosedcache management system that employs a dual-bank cache memory.

FIG. 4 is a control flow diagram for the disclosed cache managementsystem.

FIG. 5A is an arbitration control diagram for a first embodiment of thedisclosed cache management system.

FIG. 5B is a timing diagram for one conventional cache managementsystem.

FIG. 5C is a timing diagram for the cache management system of FIG. 5A.

FIG. 5D is a flowchart for the cache management system of FIG. 5A.

FIG. 6A is a timing diagram for a second embodiment of the disclosedcache management system.

FIG. 6B is a flowchart for the second embodiment of the disclosed cachemanagement system.

FIG. 7A is an arbitration control diagram for a third embodiment of thecache management system.

FIG. 7B is a timing diagram for the third embodiment of the cachemanagement system.

FIG. 7C is a flowchart for the third embodiment of the cache managementsystem.

FIG. 8A is an arbitration control diagram for the fourth embodiment ofthe cache management system.

FIG. 8B is a timing diagram for a fourth embodiment of the cachemanagement system.

FIG. 8C is a flowchart for the fourth embodiment of the cache managementsystem.

FIG. 8D continues the flowchart for the fourth embodiment of the cachemanagement system of FIG. 8C.

DETAILED DESCRIPTION

In one embodiment, the disclosed information handling system (IHS)includes a cache and directory management mechanism with an L2 store-incache that provides minimal core latency by giving load operations theability to interrupt internal L2 multi-beat store operations that arealready in progress. This provides the load operation with immediateaccess to the L2 cache and causes the interrupted store operation torecycle and proceed efficiently where it left off at the point ofinterruption. This mechanism may increase core performance by treatingcore load accesses as immediate access type operations at the expense ofdelaying or interrupting less sensitive store operations.

FIG. 1 shows one embodiment of information handling system (IHS) 100that includes a processor array 105 that employs the disclosed cache anddirectory management mechanism. Processor array 105 includesrepresentative processors 221, 222 and 223. In practice, processor array105 may include more or fewer processor than shown in FIG. 1 dependingon the particular application. Each of processors 221, 222 and 223 mayinclude multiple processor cores, i.e. processor elements. IHS 100processes, transfers, communicates, modifies, stores or otherwisehandles information in digital form, analog form or other form.

IHS 100 includes a bus 115 that couples processor array 105 to systemmemory 120 via a memory controller 125 and memory bus 130. In oneembodiment, system memory 120 is external to processor array 105. Systemmemory 120 may be a static random access memory (SRAM) array or adynamic random access memory (DRAM) array. Processor array 105 may alsoinclude local memory (not shown) such as L1 and L2 caches (not shown) onthe semiconductor dies of processors 221, 222 and 223. A video graphicscontroller 135 couples display 140 to bus 115. Nonvolatile storage 145,such as a hard disk drive, CD drive, DVD drive, or other nonvolatilestorage couples to bus 115 to provide IHS 100 with permanent storage ofinformation. Nonvolatile storage 145 provides permanent storage to anoperating system 147. Operating system 147 loads in memory 120 asoperating system 147′ to govern the operation of IHS 100. I/O devices150, such as a keyboard and a mouse pointing device, couple to bus 115via I/O controller 155 and I/O bus 160. One or more expansion busses165, such as USB, IEEE 1394 bus, ATA, SATA, PCI, PCIE and other busses,couple to bus 115 to facilitate the connection of peripherals anddevices to IHS 100. A network interface adapter 170 couples to bus 115to enable IHS 100 to connect by wire or wirelessly to a network andother information handling systems. While FIG. 1 shows one IHS thatemploys processor array 105, the IHS may take many forms. For example,IHS 100 may take the form of a desktop, server, portable, laptop,notebook, or other form factor computer or data processing system. IHS100 may take other form factors such as a gaming device, a personaldigital assistant (PDA), a portable telephone device, a communicationdevice or other devices that include a processor and memory.

FIG. 2 is a block diagram of a representative processor integratedcircuit (PROC IC) 220. Processor integrated circuit 220 includes achiplet 201, chiplet 202 . . . N, wherein N is an integer. In moredetail, chiplet 201 is a portion of an integrated circuit die thatincludes a processor core 210, an instruction fetch unit (IFU) 214, aload store unit (LSU) 211, and an instruction scheduling unit (ISU) 212.Instruction fetch unit (IFU) 214 includes an L1 instruction cachedesignated L1 I$ that couples to L2 cache system 213 via instructionload bus 218. Processor core 210 is an example of a processor element.Load store unit (LSU) 211 includes an L1 data cache designated L1 D$that couples to L2 cache system 213 via store bus 219 and load bus 218.Load bus 218 enables both the L1 data cache L1 D$ and the L1 instructioncache I$ to receive data from L2 cache system 213. Store bus 219 enablesthe LSU 211 to send data for store operations to L2 cache system 213.Load bus 218 transports load operations from IFU 214 and LSU 211 to L2cache system 213. Store bus 219 transports store operations from core210 to L2 cache system 213. Chiplet 201 further includes an L2 cachesystem 213 that couples to the instruction cache L1 I$ and to data cacheL1 D$, as shown. L2 cache system 213 couples via bus 216 to L3 cache217. The size of L3 cache memory 217 is larger than that of L2 cachememory 213. For example, in one embodiment, L2 cache memory 213 exhibitsa size of 256 KB and L3 cache memory 217 exhibits a size of 4 MB. Thesize of these cache memories may vary and is not limited to theserepresentative values. L2 cache system 213 is a unified cache in that itstores both instructions and data.

L2 cache system 213 and L3 cache 217 couple to system bus 215. Chiplets202 . . . N also couple to system bus 215. A memory controller 225couples between system bus 225 and a system memory 226 external 225 toprocessor IC 220. An I/O controller 230 couples between system bus 215and external I/O devices 227. Other processor integrated circuits 221 .. . M may couple to system bus 215 as shown. M is in integer thatrepresents the number of processors in a particular implementation.

In this particular embodiment, the L1 instruction and data caches arehigh speed memory that allow for quick access to the information in theL1 cache, such as within 3 processor clock (3 PCLK) cycles, for example.The L1 cache stores validity information indicating whether theparticular entries therein are currently valid or invalid. The L2 cachesystem 213 is a store-in cache wherein load and store operations mayexecute by using the information in the L1 cache if there is a hit inthe L1 cache. If a cache line containing the information that a load orstore operation needs is not in the L1 cache, then the L2 cache system213 is responsible to go find the coherent copy of the cache line, pullin the cache line and match the cache line up with the respective loador store operation. Processor core 210 thus does not see main memory,i.e. system memory 226, when the processor core 210 performs a load orstore operation because it directs those operations to L2 cache system213 if no hit occurs in the L1 cache.

In terms of core efficiency, execution of load operations is moreimportant than the execution of load operations in the disclosed IHS.Assume for discussion purposes that the disclosed IHS executes aprogram. While executing the program, a processor core encounters astore operation request. When the core encounters the store operationrequest it puts the store operation request in the L1 cache and sends itto the L2 cache system 213 to make it coherently visible to the rest ofthe system. However, if core 210 can not immediately execute the storerequest operation, chiplet 201 may temporarily store the store requestin a store queue (not shown in FIG. 2). Core 210 of chiplet 201 willcontinue executing instructions as long as the store queue does not fillup. However, if core 210 can not immediately execute a load operationrequest because the load operation is not available in the L1 cache,core 210 may stop and wait until the load operation request completes.Completion of this load operation request may involve retrieving theload operation from L2 cache system 213, L3 cache 217 or system memory226. It is thus more important for load operations to execute quicklythan for store operations. While load operations are latency sensitivefor performance, store operations are bandwidth sensitive forperformance. Store operations do not have a latency issue, however storeoperations do have a bandwidth aspect in the sense that since L2 cachesystem 213 sees all store operations as an incoming stream, L2 cachesystem 213 should not become backed-up. If the L2 cache system 213becomes backed-up beyond a particular point, then this back-up willnegatively impact core performance. In other words, if all store queuesfill up, the incoming stream of store operations should stop to allowalready queued store operations to process and clear.

L3 cache 217 couples to L2 cache system 213 such that requests comingfrom core 210 go first to L2 cache system 213 for fulfillment. From acoherency standpoint, core 210 exhibits 2 states, namely valid andinvalid with respect to instructions and data. In one embodiment, the L2cache system 213 exhibits a size of 256 KB and L3 cache 217 exhibits asize of 4 MB. Core 210 employs a store-through L1 cache. The L2 cachesystem 213 is a store-through cache such that L2 cache system 213 seesall store traffic. The L2 cache system 213 is the location in chiplet201 where operations such as store operations are made coherentlyvisible to the rest of the system. In other words, core 210 looks to theL2 cache system 213 to control the claiming of cache lines that core 210may need. L2 cache system 213 controls the finding of such desired cachelines and the transport of these cache lines into the L2 cache memory.L2 cache system 213 is responsible for exposing its core 210 storescoherently to the system and for ensuring that the IFU 214 and LSU 211caches remains coherent with the rest of the system. In one embodiment,the cache line size of L2 cache system 213 is 128 bytes. Other sizecache lines are also acceptable and may vary according to the particularapplication.

The disclosed cache management methodology mixes load operations in withstore operations in a manner that may increase L2 cache efficiency ofIHS 100. Under certain circumstances, load operations may interrupt thehandling of store operations by the L2 cache system 213 to provide loadoperations with more immediate access to information that core 210 needsto continue processing load operations.

FIG. 3A shows a representation of a data flow that IHS 100 may employ topractice the disclosed cache management methodology. FIG. 3A showsseveral of the structures of chiplet 201 in more detail than FIG. 2.When comparing the structures of FIG. 2 and FIG. 3, like numbersindicate like elements. More particularly, FIG. 3 shows a data flow fora chiplet 201 that includes a single bank L2 cache memory 390 in L2cache system 213. In this particular embodiment, single bank L2 cachememory 390 is a 256 KB eight (8) way associative cache that employs 128byte cache lines. Core 210 couples to L2 cache system 213 as shown. LSU211 of core 210 includes a store queue (STQ) 309 that couples to an L2store queue buffer 310 in L2 cache system 213. Store queue 309cooperates with L2 store queue buffer 310 to supply L2 cache memory 390with store operation requests. L2 cache system 213 determines if L2cache memory 390 currently stores information that core 210 needs toexecute a load or store operation. L2 cache system 213 efficientlyarbitrates and intermixes load operations among store operations in amanner whereby a load operation may interrupt a store operation withinthe L2 cache. This action more quickly provides core 210 withinformation that core 210 needs to complete a load operation.

Core instruction load request bus 370A couples IFU 214 of core 210 to L2cache system 213 to enable core 210 to send a load instruction requestto L2 cache system 213 to bring in a requested instruction or code. Coredata load request bus 370B couples LSU 211 to the L2 cache system 213 sothe LSU 211 can send a load request to access the data that the LSUneeds to perform the task that an instruction defines. Busses 370A and370B together form load request bus 370. Core store bus 350 connectsstore queue (STQ) 309 of the LSU 211 in core 210 to L2 store queuebuffer 310. Core store bus 350 enables store operation requests to enterL2 cache system 213 from store queue 309 of core 210. Such core storerequests travel from store queue (STQ) 309 via core store bus 350 to theL2 store queue buffer 310. The L2 store queue buffer 310 packs togetherstore requests, for example sixteen consecutive 8 byte store requests.In this manner, L2 cache 213 may perform one cache line installoperation rather than sixteen. A core reload bus 360 couples a corereload multiplexer (MUX) 305 to the L1 instruction cache I$ and the L1data cache D$ of core 210.

It takes multiple processor cycles, or P clocks (PCLKs), to processloads or stores through L2 cache system 213. In this particularembodiment, L2 cache memory 390 exhibits a size of 256 KB and employs acache line size of 128 bytes. L2 cache memory 390 includes a cache writesite or write input 390A and a cache read site or read output 390B.Busses into and out of L2 cache memory each exhibit 32 bytes. Since L2cache memory 390 employs 128 byte cache lines, it takes 4 processorcycles (P clocks) to write information to L2 cache memory 390 and 4processor cycles to read information from L2 cache memory 390.

There are different reasons why L2 cache system 213 may do a cache reador a cache write, for example in response to a load or store requestcoming down to the L2 cache from core 210. If core 210 sends L2 cachesystem 213 a load or store and L2 cache memory 390 does not contain acache line that the load or store requires, then we have an L2 cachemiss. In the event of an L2 cache miss, L2 cache system 213 must findthe cache line needed by that load or store and install that cache linein L2 cache memory 390, thus resulting in a cache write. Read claim (RC)state machines RC0, RC1, . . . RC7 cooperate with RCDAT buffer 320 toretrieve the desired cache line and install the desired cache line in L2cache memory 390. The desired cache line includes the designatedinformation that the load or store from core 210 specifies. Reloadmultiplexer 305 also sends this designated information via core reloadbus 360 to the L1 cache of core 210 so that core 210 may complete theload or store.

An error correction code generator (ECCGEN) 391 couples to the writeinput 390A of L2 cache memory 390 to provide error correction codes tocache line writes of information to L2 cache memory 390 that result fromload or store requests. An error correction code checker (ECCCK) 392couples to the read output 392 of L2 cache memory 390 to check the errorcodes of cache lines read from cache memory 390 and to correct errors insuch cache lines by using error correction code information from the L2cache memory 390.

When core 210 sends a store operation to L2 cache system 213, L2 storequeue buffer 310 packs or compresses this store operation with otherstore operations. Assuming that there was a hit, then the informationthat the store operation requires is present in L2 cache memory 390. L2cache system 213 pulls the cache line that includes the designated storeinformation out of L2 cache memory 390. ECCCK circuit 392 performs errorchecking and correction on the designated cache line and sends thecorrected store information to one input of a two input store byte mergemultiplexer 355. The remaining input of store byte merge multiplexer 355couples to L2 store queue buffer 310. When L2 cache system 213determines that there is an L2 cache hit for a store operation comingout of the L2 store queue buffer 310 at MUX input 355A, L2 cache system213 pulls the information designated by that store operation from L2cache 390. This designated information appears at MUX input 355B aftererror correction. Store byte merge MUX 355 merges the information on itsinputs and supplies the information to read claim data (RCDAT) buffer320. RCDAT buffer 320 operates in cooperation with RC (read claim) statemachines RC0, RC1, . . . RC7 that control the operation of L2 cachesystem 213.

The function of a read claim (RC) state machine such as machines RC0,RC1, . . . RC7 is that, for every load or store that core 210 providesto L2 cache system 213, an RC machine will merge the data for thatstore, go find the data which is the subject of the store, and claim thecache line containing the store. The RC machine is either conducting aread for a store operation or claiming the data that is the subject ofthe store operation, namely claiming the desired cache line containingthe target of the store operation. The RC machine cooperates with theRCDAT buffer 320 that handles the transport of the desired cache linethat the RC machine finds and claims. Each RC machine may independentlywork on a task from the core, for example either a load or store requestfrom core 210, by finding the cache line that the particular load orstore requests needs. The desired cache line that the RC machine seeksmay exist within the L2 cache system 213, the L3 cache (not shown inFIG. 3) connected to L3 bus 216 or in system memory (not shown in FIG.3) coupled to system bus 215. The RC machine looks first in L2 cachememory 390 for the desired data. If the L2 cache memory 390 does notstore the desired data, then the RC machine looks in the L3 cachecoupled to L3 cache bus 216. If an L3 hit occurs, then the RC machinesinstructs MUX 332 to transfer the desired cache line, i.e. the L3 hitdata, from L3 bus 216 to RCDAT buffer 320. Reload MUX 305 then passesthe L3 hit data via core reload bus 360 to core 210 and then to L2 cachememory 390 via ECC generator 391.

If the RC machine does not find the desired cache line in the L3 cache,then an L3 miss condition exists and the RC machine continues lookingfor the desired cache line in the system memory (not shown) that couplesto system bus 215. When the RC state machine finds the desired cacheline in system memory, then the RC machines instructs MUX 332 totransfer the desired cache line from system bus 215 to RCDAT buffer 320.Reload MUX 305 then passes the desired cache line via core reload bus360 to core 210 and then to L2 cache memory 390 via ECC generator 391.

RCDAT buffer 320 is the working data buffer for the 8 RC state machinesRC0, RC1, . . . RC7. RCDAT buffer 320 is effectively a scratch padmemory for these RC state machines. RCDAT buffer 320 provides 128 bytesof dedicated storage per RC state machine. Thus a different 128 bytecache line may fit in each of RC state machines RC0, RC1, . . . RC7.

In the case of an L2 cache hit, L2 cache system 213 pulls the designatedinformation out of L2 cache memory 390. If the store operation is for astore operation from the core 210, one of the read claim (RC) machinesis responsible for finding that line either in the L2 cache orelsewhere, merging the found designated line at store byte merge buffer355 if the RC machine finds the designated line in the L2 cache, ormerging the found designated line in the RCDAT buffer 320 if the RCmachine does not find the designated line in the L2 cache. Once the RCmachine completes the installation of the merged line in RCDAT buffer320, then it puts the designated line back in the L2 cache memory 390.

Once an operation is in the RCDAT data buffer 320, if that operation isa store operation, then the RCDAT data buffer 320 needs to write thatoperation back into L2 cache memory 390, as described above. However, ifthat operation in the RCDAT data buffer 305 is a load operation andthere is a hit in the L2 cache, then the load operation takes a paththrough store byte merge MUX 355 similar to the case of the storeoperation described above. However, in the case of a load operation hitin the L2 cache, the designated hit cache line in L2 cache memory 390passes through MUX 355 with no merge operation and goes into RCDATbuffer 320 for storage. The designated hit cache line for the loadoperation then travels directly to core 210 via reload MUX 305 and corereload bus 360. By “directly” here we mean that the designated hit cacheline for the load passes from RCDAT buffer 320 to core 210 withoutpassing through ECC generator 391 and its associated delay. However, iferror checker 392 determines that the designated hit cache line found inL2 cache memory 390 does exhibit an error, then error checker 392corrects the error and places the corrected cache line in RCDAT buffer320. In response, RCDAT buffer 320 redelivers the cache line, nowcorrected, to core 210.

L2 cache system 213 includes a cast out/snoop (CO/SNP) buffer 325 thatcouples between the read output 390B and L3 bus 216 and system bus 215as shown. As cache lines write to L2 cache memory 390, old cache lineswithin L2 cache memory 390 may need removal to make room for a newercache line. In this situation, a cast out state machine, discussed inmore detail below, selects a victim cache line for expulsion from cachememory 390. The cast out state machine instructs CO/SNP buffer 325 tosend the old cache line, namely the victim cache line, to the L3 cache(not shown) via L3 bus 216. The CO/SNP buffer 325 also couples to systembus 215 to enable the transport of victim cache lines to system memory(not shown) that couples to system bus 215. The L2 cast out data outputof CO/SNP buffer 325 couples to L3 bus 216 and system bus 215 for thispurpose. The CO/SNP buffer 325 also couples to system bus 215 to enablea snoop state machine (not shown in FIG. 3A) to allow other processorIC's such as processor IC 221 to snoop the cache line contents of L2cache memory 390 as needed.

First and second embodiments of the disclosed cache managementmethodology may employ the single bank L2 cache configuration that FIG.3A depicts. Third and fourth embodiments may employ the dual bank L2cache configuration that FIG. 3B depicts. FIG. 3B is similar to FIG. 3Aexcept for the dual bank L2 cache 390′ architecture that FIG. 3Bemploys. Like numbers indicate like elements when comparing FIG. 3B withFIG. 3A. L2 cache memory 390′ includes 2 banks of high speed cachememory, namely BANK0 and BANK1. BANK0 is a 256 KB eight (8) way setassociative cache with 128 B cache lines. Likewise, BANK1 is a 256 KBeight (8) way set associative cache with 128 B cache lines. BANK0 storeseven cache lines while BANK1 stores odd cache lines. If a particularcache line exhibits a least significant bit (LSB) that is 0, then thatcache line is even and L2 cache 390′ stores that even cache line inBANK0. However, if a particular cache line exhibits an LSB that is 1,then that cache line is odd and L2 cache 390′ stores that odd cache linein BANK1. In this manner, it is possible to read from one bank whilewriting to the other. L2 cache memory 390′ includes a read output 390Cthat supplies even cache lines to one input of a two input multiplexer395. The remaining input of multiplexer 395 couples to a read output390D that supplies odd cache lines. Multiplexer 395 can select either aneven cache line from BANK0 or an odd cache line from BANK1 of L2 cachememory 390′.

While FIGS. 3A and 3B describe data flows for the disclosed L2 cachemanagement apparatus and methodology, FIG. 4 shows a representativecontrol flow for the structures of FIGS. 3A and, 3B. The control flowthat FIG. 4 depicts controls the mechanisms and structures of FIGS. 3Aand 3B that carry out the disclosed cache management methodology. It ishelpful to conceptually view the control flow of FIG. 4 as beingsuperimposed on top of the data flow of FIG. 3A, or alternatively, ontop of FIG. 3B. In many cases, the state machines and other controlstructures that FIG. 4 depicts may map to, or correspond to, respectivestructures within the L2 cache system 213 of FIGS. 3A and 3B. Forconvenience, the following discussion will relate the control flow ofFIG. 4 with the single-bank L2 cache memory architecture of FIG. 3A,although the discussion is applicable as well to the dual-bank L2 cachememory architecture of FIG. 3B.

To help relate the control flow of FIG. 4 with the data flow of FIG. 3A,in many instances the elements in the control flow of FIG. 4 arenumbered such that the last two digits correspond to the last two digitsof the corresponding controlled structure within the data flow of FIG.3A. For example, store queue control logic 410 of FIG. 4 controls theoperation of L2 store queue buffer 310 of FIG. 3A. FIG. 4 depicts L2cache memory 390 using the same number in FIG. 4 and FIG. 3A.

The control flow of FIG. 4 includes 1) state machines, 2) generalcontrol logic and 3) arbiters. The L2 cache system 213 of FIG. 4includes a cache arbiter (CACHE ARB) 420 that schedules reads and writesin the L2 cache memory 309. Loads and stores coming from core 210 formthese reads and writes. L2 cache system 213 of FIG. 4 includes a CPUdirectory arbiter (CPU DIR ARB) 421 that controls access to theCPU/snoop directory (CPU/SNP DIR) 491. CPU directory arbiter 421controls access to the directory 491 “to the north”, i.e. between L2cache system 213 and core 210. Directory 491 stores address and stateinformation for all cache lines in L2 cache memory 390. This stateinformation may include the MESI state information for each cache line,namely “modified”, “exclusive”, “shared” or “invalid”. While L2 cachememory 390 physically holds the data, directory 491 holds the addressthat associates with the individual pieces of data that the L 2 cachememory 390 stores. Snoop directory arbiter (SNP DIR ARB) 422 controlsaccess to the directory 491 “to the south”, i.e. between L2 cache system213 and system bus interfaces 215.

Core 210 sends requests, i.e. loads and stores, to L2 cache system 213for handling. Loads enter core interface unit control (CIU) logic 441from core load request bus 370. Stores enter store queue control logic410 from core store bus 350. As these load and store requests come infrom core 210, CPU directory arbiter (CPU DIR ARB) 421 arbitratesbetween the load and store requests and sends the resultant arbitratedload and store requests to RC dispatch control (RC DISP CONTROL) logic404. RC dispatch control logic 404 sends or dispatches these requests toa read claim (RC) state machine 401 or a cast out (CO) state machine402, as appropriate. In one embodiment, eight (8) RC state machines areavailable and eight (8) CO state machines are available to handle suchdispatches. If a store operation results in the need for a victim, acast out state machine 402 determines the particular victim. The castout state machine 402 expels the victim cache line and sends the victimcache line to L3 interface 216 for storage in the L3 cache. In moredetail, L3 control logic (L3CTL) 432 is an address arbiter that handlescast out requests and sends the victim cache line to the L3 cache forstorage. In the data flow of FIG. 4, WR designates a write operation andRD designates a read operation.

When an RC state machine 401 handles a load or store that involves aparticular cache line, the RC state machine 401 first searches L2 cachememory 390 to see if L2 cache memory 390 contains the particular cacheline. As seen by the line exiting the bottom of RC state machine 401 inFIG. 4, if the RC state machine does not find the particular cache linein L2 cache memory 390, then the request either goes to system bus 215via the system bus arbiter (SB ARB) 430 or it goes to the L3 cache viaL3 cache interface 216 as a read claim request (RC REQ). To summarize,when a load or store comes into an RC machine 401, the RC machine firstlooks in the L2 cache memory 390. If the cache line that the load orstore request designates is not in the L2 cache memory 390, then the RCmachine 401 sends the request to the L3 cache via the L3 interface bus216. If the L3 cache responds back that the designated cache line forthe request is not in the L3 cache, then the RC request goes throughsystem bus arbiter (SB ARB) 430 out the system bus 215 to system memory.

L2 cache system 213 includes reload bus control logic 405 for deliveringcache lines back to core 210 via core reload bus 360. Reload bus controllogic 405 of the control flow of FIG. 4 controls reload MUX 305 of thedata flow of FIG. 3A.

Other processor ICs on system bus 215 such as processor IC 221 may needto look in directory 491 to determine if L2 cache memory 390 contains acache line that processor IC 221 needs. Processor IC 221 may send asnoop request over system bus 215 requesting this information. Snoopdirectory arbiter (SNP DIR ARB) 422 receives such a snoop request. Inpractice, this snoop request may originate in an RC state machine ofanother processor IC. System bus 215 may effectively broadcast the snooprequest to all processor ICs on the system bus. If snoop directoryarbiter 422 determines that L2 cache memory 390 contains the cache linerequested by the snoop request, then SNP DIR ARB 422 dispatches intofour snoop (SNP) state machines 403 as seen in FIG. 4. Snoop statemachines 403 manage the reference and protection of requests forownership by other caches via system bus 215. Snoop state machines 403communicate with system bus 215, reload bus control logic 405, directory491 and cache arbiter 420 during this process. Each of the statemachines 403 may perform a different cache line task. For example, onestate machine 403 may kill the cache line that the snoop requestdesignates because the cache line changed in another processor IC.Another task that a SNP state machine 403 may perform on the cache lineis to send the cache line to system memory via system bus interface 215.Yet another task that an SNP state machine 403 may perform is to sendthe cache line to another processor IC such as 221 the requests thecache line.

L2 cache system 213 includes a system bus arbiter (SB ARB) 430 forhandling commands and a data out control (DOCTL) data arbiter 431 whichacts as a data arbiter. DOCTL data arbiter 431 issues data requests tosystem bus 215 on behalf of cast out state machines 402 and snoop statemachines 403 to move data to system bus 215. Snoop requests that L2cache system 213 receives from system bus 215 may require two actions,namely sending a command to a snoop state machine and setting up acommunication with another cache or another processor IC. SB arbiter 430issues data requests to system bus 215 on behalf of RC state machines401, cast out state machines 402 and snoop state machines 403.

The L2 cache memory is inclusive of the contents of the L1 cache in theprocessor core 210. This means that all lines in the L1 cache are alsoin the L2 cache memory 390. When the L2 cache system detects a change ina particular cache line, for example by detecting a store operation onsystem bus 215, the L2 cache system sends an “invalidate” notice (INV)to the L1 cache in the processor core to let the L1 cache know that theL1 cache must invalidate the particular cache line. FIG. 4 shows suchinvalidate notices as INV. Normally CPU directory arbiter 421, cachearbiter 420 and snoop directory arbiter 422 work independently toservice individual requests from the busses and machines they support.But when directory arbiter 421 is dispatching a load or store to the RCmachine 401, the CPU directory arbiter 421 and the cache arbiter 420interlock such that the data reads immediately out of the L2 cachememory 390 in the case of a L2 cache hit. In this way, the CPU directoryarbiter 421 and cache arbiter 420 interlock, as arbiter interlock line423 indicates, and work in conjunction to perform given high prioritytask such as load and store dispatch requests.

FIG. 5A is an arbitration control diagram for a first embodiment of thedisclosed cache management methodology. The data flow diagram of FIG. 3Aand the control flow diagram of FIG. 4 both apply to this firstembodiment. The control diagram of FIG. 5A provides more detail withrespect to particular arbitration aspects of the control flow diagram ofFIG. 4 as L2 cache 215 conducts the disclosed cache managementmethodology of the first embodiment.

The first embodiment of FIG. 5A relates to an L2 store-in cache anddirectory control management methodology with immediate scheduling ofcore loads. This cache methodology achieves minimal core load latency byproviding core load operations with the ability to interrupt multi-beataccesses such as store operations that are already in progress in an L2cache. If the L2 cache commences servicing a store request operationfrom a processor core, the L2 cache allows a load operation from aprocessor core to interrupt the store operation already in process. TheL2 cache immediately services the load operation. Once servicing of theload operation is complete, the L2 cache returns to handling theinterrupted load request at the point of interruption of the storerequest.

To appreciate the operation of the first embodiment, a comparisonbetween a timing diagram for the cache management method of the firstembodiment and a timing diagram from one conventional cache managementmethod is helpful. FIG. 5B is a timing diagram that depicts theoperation of one conventional cache management method. The horizontalaxis represents time, namely 20 processor clock cycles or P-clock (PCLK)cycles. Rounded rectangular boxes depict load operations that theconventional L2 cache handles, i.e. cache accesses for a core interfaceunit (CIU). Circles or ovals indicate store operations for a storequeue, i.e. cache accesses for a store queue.

The L2 cache receives a load operation request and performs therequested load operation in cache accesses CO-A, CO-B, CO-C and CO-Dduring cycles 3, 4, 5 and 6 respectively. At the end of this loadoperation and at the request of the core, the L2 cache commences a storeoperation. The L2 cache performs the requested store operation in cacheaccesses SO-A, SO-B, SO-C and SO-D during cycles 7, 8, 9 and 10,respectively. In cycle 9, the L2 cache receives another request, namelya load request. However, the L2 cache can not service the load requestbecause it is still working on the previous store request in cycles 9and 10. The L2 cache waits until servicing of the store request iscomplete at cycle 10 and then commences servicing the load request atcycle 11. The L2 cache performs the requested load operation in cacheaccesses C1-A, C1-B, C1-C and C1-D during cycles 11, 12, 13 and 14,respectively. The X's in the boxes in cycles 9 and 10 represent thedelay in servicing the second load request that the previous storerequest causes.

FIG. 5C shows a representative timing diagram for the L2 cachemanagement methodology that the first embodiment employs. The L2 cachereceives a load operation request and performs the requested loadoperation in cache accesses CO-A, CO-B, CO-C and CO-D during cycles 3,4, 5 and 6 respectively. At the end of this load operation and at therequest of the core, the L2 cache commences a store operation. The L2cache performs the requested store operation in cache accesses SO-A andSO-B during cycles 7 and 8, but receives an interruption from anotherload request. The L2 cache interrupts the pending store operation andimmediately starts servicing the load request. The L2 cache performs therequested load operation in cache accesses C1-A, C1-B, C1-C and C1-Dduring cycles 9, 1 0, 11 and 12, respectively. Once servicing of theinterrupting load operation is complete at cycle 12, the L2 cachereturns to servicing the interrupted store operation at the point ofinterruption and continues with cache accesses SO-C and S0-D to completethe store operation during cycles 13 and 14, respectively. The firstembodiment of FIG. 5C thus substantially reduces load latency incomparison with the L2 cache methodology of FIG. 5B.

Returning to the arbitration control diagram of FIG. 5A, the arbitrationthat occurs in the first embodiment is now discussed. FIG. 5Aeffectively enlarges or concentrates on portions of the control flow ofFIG. 4. For example, the control diagram of FIG. 5A shows more detailwith respect to cache arbiter 420 and directory arbiter 421. FIG. 5Aalso depicts core interface unit (CIU) 441 in the load path and storequeue 410 in the store path.

The purpose of FIG. 5A is to depict the arbitrations that occur toobtain access to L2 cache 390 and directory 491 shown at the bottom ofFIG. 5A. One goal of these of these arbitrations is to effectively getthe load and store operations from the core together in a line becausesingle-bank L2 cache 390 can only do one operation at time. The depictedcontrol diagram arbitrates to arrange the loads and stores in such afashion that a load may interrupt a store operation in the L2 cache andthe L2 cache may continue servicing the interrupted store at the pointof interruption once the interrupting load operation completes.

RC07 is a shorthand notation for state machines RC0, RC1 . . . RC7. CO07is a shorthand notation for cast out state machines CO-0, CO-1, . . .CO7. SN03 is a shorthand notation for snoop machines SN0, SN1, . . .SN3. When any of these RC state machines, CO state machines or snoopmachines need to access L2 cache 390 or directory 491, they need to gothrough the stage 1, stage 2 and stage 3 arbitrations shown in FIG. 5A.Cache arbiter 420 conducts an 8 way arbitration among the 8 RC statemachines RC07. The designation ARB8 in the oval adjacent RC07 signifiesthis 8 way arbitration. Cache arbiter 420 also conducts an 8 wayarbitration among the 8 cast out state machines CO07. Cache arbiter 420further conducts a 4 way arbitration ARB4 among the 4 snoop statemachines SN03. The result of these 3 arbitrations feeds a 3 wayarbitration ARB3 as shown in cache arbiter 420 of FIG. 5A. These RC07,CO07 and SN03 arbitrations, followed by the arbitration of the 3 resultsof these arbitrations, are all “stage 1” arbitrations. Stage 2arbitration follows stage 1 arbitration and stage 3 arbitration followsstage 2 arbitration as discussed below.

Store queue control logic 410 performs a 16 way arbitration (ARB16) at510. This corresponds to an 8 way arbitration to load up store queuebuffer 310 and an 8 way arbitration to unload this store queue buffer.In other words, ARB16 at 510 is actually two 8 way arbitrations. Thesetwo 8 way arbitrations are stage 1 arbitrations as shown in FIG. 5A. Inthis manner, L2 store queue buffer 310 receives a supply of storeoperations to execute or service during stage 1. Ultimately, after the16 arbitrations at 510, a single result of this arbitration appears asone input to a 2 way arbitration (ARB2) at 526 in a stage 2. The otherinput to this 2 way arbitration (ARB2) is the result of the earlier 3way arbitration in cache arbiter 420. The result of this two wayarbitration in stage 2 becomes one input of a 2 way arbitration (ARB2)at 527 in a stage 3 that follows stage 2, as shown. Also during stage 1,core interface unit control logic 441 conducts an 8 way arbitration(ARB8) at 541 to determine the load instruction that should proceed tothe next stage. The remaining input of this 2 way arbitration 527receives the load request result of the 8 way arbitration that CIUcontrol 441 conducted. The output of the two way arbitration (ARB2) at527 supplies arbitration results to sequencer 528. These results includeload requests, store requests and other requests.

In summary, many requests contend for access to the L2 cache 390. Thesecontending requests includes load requests from CIU control 441, storerequests from store queue control 410, as well as requests from the RCstate machines RC07, the cast out state machines CO07 and the snooprequest state machines SN03. The arbiters process these requests inparallel to pick a winner to go to a subsequent stage. The stage 2arbitration encompasses all of the state machines listed above. Thestage 3 arbitration is the final arbitration that selects the currentrequest for the L2 cache to process.

The control diagram of FIG. 5A also shows contention for directory 491by core interface unit 441 (for loads), store queue 410 (for stores) andthe RC state machines RC01, the cast out machines CO07 and the snoopstate machines SN03. Read claim machines RC07 go to directory arbiter421 to do writes to directory 491. FIG. 5A shows a blow-up of directoryarbiter 421. Directory arbiter 421 includes an arbitration 521 with an 8way arbitration (ARB8) for RC machines RC07 and a 4 way (ARB4)arbitration for the snoop state machines SN03. This occurs because boththe RC machines RC07 and the snoop machines SN03 may desire to performan update of directory 491. A two way arbitration (ARB2) arbitratesbetween the result of the 8 way arbitration (ARB8) for the RC statemachines and the result of the 4 way arbitration (ARB4) for the snoopmachines, as seen in FIG. 5A. The result of this arbitration (ARB2) goesto a 3 way arbitration (ARB3) at 529. The result of the 8 wayarbitration (ARB8) at 541 in core interface unit control logic 441 goesto the 3 way arbitration (ARB3) at 529 for directory 491. This accountsfor 2 of the 3 inputs to 3 way arbitration (ARB3) at 529. The result ofthe two way arbitration (ARB2) at 526, discussed above, provides thethird input to the 3 way arbitration (ARB3) at 529. The winner of the 3way arbitration (ARB3) at 529 receives access to directory 491. Loadsfrom CIU control logic 441 in the load path receive immediate access todirectory 491 without any intervening arbitrations, except for the 3 wayarbitration (ARB3) at 529. A load operation will win the 3 way (ARB3)arbitration at 529 and receive immediate access to the directory 429ahead of the requests from competing requesters such as RC statemachines, cast out state machines, snoop state machines and store queue410.

In the control diagram of FIG. 5A, loads exhibit a lower latency thatstores. Loads from the 8 way arbitration (ARB8) at 541 from stage 1 godirectly to the 2 way arbitration (ARB2) at 527 in stage 3, thusbypassing stage 2 arbitration. A load at the 2 way arbitration (ARB2) at527 prevails over a competing store or other request. Such a loadrequest passes immediately from stage 3 to L2 cache 390 for expeditedservicing, thus taking precedent over any currently executing storeoperation. A load request will thus interrupt a currently executingstore operation. When the interrupting load operation completes, the L2cache will continue processing the interrupted store operation from thepoint of interruption.

FIG. 5D is a high level flowchart that depicts process flow in the firstembodiment of the disclosed L2 cache management methodology. Processflow commences at start block 540. L2 cache system 213 receives load andstore requests from core 210. L2 cache system 213 performs a test todetermine if a particular request that it receives is a load request, asper decision block 545. If the particular request is a load request,then L2 cache system performs another test to determine if the L2 cacheis currently busy on another load request, as per decision block 550. Ifthis test determines that the L2 cache is currently busy handlinganother load request, then L2 cache system 213 keeps recycling testblock 545 and test block 550 until test block 550 determines that the L2cache is no longer busy handling another load request. When the L2 cacheis no longer busy handling another load request, then the L2 cachestarts an L2 cache access to service the load request, as per block 555.In this first embodiment, load requests receive priority over storerequest with respect to accessing the L2 cache memory 390. In L2 cachesystem 213, load requests receive priority handling over store requests.Moreover, load requests may interrupt store request accesses that arealready underway. After servicing an interrupting load request, L2 cachesystem 213 may return to servicing the interrupted store request at thepoint of interruption of the store request.

If the test at decision block 545 determines that the particular requestis not a load request, then L2 cache system 213 tests to determine ifthe particular request is a store request, as per decision block 560. Ifthe particular request is not a store request, then process flowcontinues back to the load request test at decision block 545. However,if the particular request is a store request, then L2 cache system 213starts an L2 cache memory access to service the store request, as perblock 565. L2 cache system 213 then conducts a test to determine if thestore request completed a cache line access, namely a store or writeoperation, as per block 570. If the store request completed a cache lineread, then process flow continues back to decision block 545 to monitorfor more incoming load requests. However, if the store request did notyet complete a cache line read to completely fulfill the request, thenL2 cache system 213 conducts a test to determine if L2 cache system 213now receives a load request for access to cache memory 390, as per block575. If the received request is a load request, then the L2 cache system213 conducts a further test to determine if the cache memory 390 is busywith another load request access, as per block 580. If the L2 cachesystem is not already busy servicing another load request, then the L2cache system is currently servicing a store request. L2 cache system 213interrupts the servicing of this store request and commences servicingthe received load request instead, as per block 585. In this scenario,the load request is an interrupting load request and the store requestis an interrupted store request. L2 cache system 213 starts an L2 cachememory access to service the interrupting load request, as per block590.

If the test at decision block 575 determines that the current requestreceived is not a load request, then L2 cache memory system 213 proceedswith the current store cache access or restarts the interrupted storecache access at the point of interruption, as per block 595. If the testat decision block 580 determines that the L2 cache is currently busyhandling a load request, then L2 cache memory system 213 proceeds withservicing the current load request, as per block 595.

In this first embodiment, a load or store operation that needs the L2cache may consume four (4) beats or cycles (PCLKs). Other embodimentsare possible where a load or store operation may consume a differentnumber of beats. Control logic in the L2 cache system may interrupt astore operation on any one of the 4 beats, i.e. a variable number ofbeats or cycles depending on the particular application. For example, ifa load operation reaches the L2 cache system at the second beat of astore operation, the L2 cache system may interrupt the store operationin progress and immediately start servicing the interrupting loadoperation at the second beat. Later, after completion of servicing theinterrupting load operation, the L2 cache may return to service theremaining 3 beats of the interrupted store operation.

FIG. 6A depicts a timing diagram for a second embodiment of thedisclosed cache management methodology. The second embodiment exhibits anumber of similarities to the first embodiment. Like the firstembodiment, the second embodiment is a cache methodology that achievesminimal core load latency by providing core load operations with theability to interrupt multi-beat accesses such as store operations thatare already in progress in a single-bank L2 cache. However, in thesecond embodiment, the disclosed cache management methodology provides astore operation with fine-grained access size to the L2 cache. Becausestore requests from the core in many benchmarks involve less than a fullcache line, there is a performance and power benefit to allowing storesto only access the cache specifically for the bytes and cycles that thestore request actually needs rather than accessing and reading out theentire cache line. This may reduce power consumption by limiting thenumber of L2 cache cycles that a store operation consumes. This may alsoprovide an increased effective bandwidth for important load accesses bythe core.

The second embodiment employs substantially the same arbitrationmechanism that arbitration control diagram FIG. 5A depicts. Not allstore operations from the core are 128 bytes, i.e. the cache line size.For example, the core may send a single 4 byte store operation requestto the L2 cache system that the L2 cache system may merge into a 128 Bcache line. However, performance increases if the L2 cache limits storesto accessing the L2 cache memory for the particular bytes and cyclesthat the store request actually needs, rather than the entire cacheline.

FIG. 6A is a timing diagram that illustrates the operation of the L2cache system of the second embodiment. The L2 cache receives a loadoperation request and performs the requested load operation in cacheaccesses CO-A, CO-B, CO-C and CO-D during cycles 3, 4, 5 and 6respectively. At the end of this load operation and at the request ofthe core, the L2 cache commences a store operation. However, this storeoperation does not consume an entire 128 B cache line and just requires2 beats of processor clock (PCLK) cycles to complete. The L2 cachesystem, or more specifically the store queue control 410 of FIG. 4 (or510 of FIG. 5A,) tracks the size requirements of this store request suchthat sequencer 528 of FIG. 5A knows that this particular store operationneeds just 2 beats, i.e. a reduced number of beats in comparison to whatanother store request may need. The size requirement of a particularstore operation corresponds to the size in terms of the number of beatsor cycles of L2 cache memory that the particular store operationrequires. Stated alternatively, the size requirement of a particularstore operation corresponds to the size in terms of the minimum numberof bytes from a cache line that the store operation requires to access.The size requirement may thus be a minimum size requirement. The L2cache performs the requested store operation in cache accesses SO-A andSO-B during cycles 7 and 8. The L2 cache access for this store operationis now complete and the L2 cache is ready for another load or storeoperation. At the end of this short store operation, the L2 cache systemreceives another load operation request. The L2 cache system performsthe requested load operation in cache accesses C1-A, C1-B, C1-C and C1-Dduring cycles 9, 10, 11 and 12, respectively. The L2 cache is thenavailable for servicing other requests.

FIG. 6B is a high level flowchart that depicts process flow in thesecond embodiment of the disclosed L2 cache management methodology. Theflowchart of FIG. 6B exhibits many similarities to the flowchart of FIG.5D discussed above. Like numbers indicate like steps when comparing theflowcharts of FIG. 6B and FIG. 5D. One difference in the flowchart ofFIG. 6B is that after L2 cache management system 213 tests anddetermines that the currently received request is a store request atdecision block 560, the L2 cache management system 213 determines thesize of the store request, as per block 605. In other words, system 213determines the number of beats or the number of bytes that a particularstore request requires to obtain the data it needs. This number of beatsor bytes may be less than the number of beats or bytes that correspondto an entire cache line.

L2 cache management system 213 begins a cache access to execute thestore request, as per block 565. System 213 conducts a test to determineif the system completed a store-sized write operation, as per decisionblock 570′. In other words, decision block 570′ determines if the storerequest already wrote to the portion of the L2 cache line that it needsto execute as opposed to accessing the entire cache line. If decisionblock 570 finds this to be true, then process flow continues back todecision block 545 where monitoring for load requests begins again. Thisaction speeds up the processing of store requests because cachemanagement system 213 does not access the entire cache line when itexecutes a store operation, but rather accesses the portion of the cacheline that it needs.

If L2 cache management system 213 determines at decision block 570′ thatthe store request did not complete a store-sized read access, thensystem 213 continues accessing cache memory 390 for the store request.System 213 tests to see if an incoming request is a load request atdecision block 575. If a received request it is load request and the L2cache is not busy on another load request, then L2 cache system 213interrupts the store request being serviced and starts servicing theinterrupting load request, as per block 585. System 213 starts a cachememory 390 access to service the interrupting load request on cache bankload needs, as per block 590′. Flow then continues back to receive loadrequest decision block 575 and the process continues.

The cache and directory arbitration control diagram of FIG. 5A appliesto this second embodiment of FIGS. 6A-6B. The second embodiment employssubstantially the same arbitration mechanism that arbitration controldiagram FIG. 5A depicts. As discussed above, some store operations mayrequire substantially fewer bytes than an entire 128 byte long cacheline. Store queue control logic 410 determines and tracks the number ofcycles or beats that each store request will take to perform by the L2cache system 213. Store queue control logic 410 of FIG. 5A incooperation with L2 store queue buffer 310 of FIG. 3A performs thistracking and determination of store time requirements. Store queuebuffer 310 gathers store operations and packs them together forforwarding to the L2 cache system for handling and completion. Thearbitration operations of FIG. 5A determine which store operation andwhich load operation the L2 cache may currently service whileimplementing the disclosed methodology that FIGS. 6A and 6B depict.These arbitration operations ultimately feed into the sequencer 528 thatcontrols the sequence of operations that the L2 cache system feeds tothe L2 cache memory for execution.

In summary, in the second embodiment, if the L2 cache system accessesthe L2 cache memory on behalf of a store operation that requires fewercycles or PCLKs than a predetermined maximum number of cycles, the storeoperation ceases after the required cycles complete rather thancontinuing up to the maximum number of cycles. In this manner, storeoperations may finish more quickly and while staying out of the way ofmore important load operations. The L2 cache mechanism accesses justthose bytes that it needs to carry out the requested store operationrather than accessing more bytes than needed and consuming more cyclesthan required.

The third embodiment employs the dual bank cache architecture that FIG.3B depicts. FIG. 3B is similar to FIG. 3A except for the dual bank L2cache 390′ architecture that FIG. 3B employs and the arbitration controlmechanism of FIG. 7B. As discussed above, L2 cache memory 390′ includes2 banks of high speed cache memory, namely BANK0 and BANK1, and a singledirectory 491. BANK0 stores even cache lines while BANK1 stores oddcache lines. Multiplexer 395 can select either an even cache line fromBANK0 or an odd cache line from BANK1 of L2 cache memory 390′.

The third embodiment employs dual data interleaving in BANK0 and BANK1of L2 cache memory 390′. The arbitration control mechanism of FIG. 7Bmay access BANK0 for a read operation at substantially the same timethat the mechanism accesses BANK1 for a write operation. This providesincreased bandwidth into and out of the L2 cache. The arbitrationcontrol mechanism may also access BANK0 for a write operation atsubstantially the same time that the mechanism accesses BANK1 for a readoperation. In other words, the arbitration control mechanism and dualcache bank architecture enables concurrent write to one cache bank whilereading from the other cache bank. While the third embodiment doesprovide for reading and writing from the dual bank L2 cache memory 390′at substantially the same time, the read and write operations may notcommence at the same time. For example, writing to one bank may beginone cycle or beat after reading begins from the other bank. However, thelater discussed fourth embodiment of FIG. 8A provides a dual bank L2cache wherein read and write operations to the two L2 cache banks maybegin at the same time.

Comparing the arbitration control mechanism of FIG. 5A with thearbitration control mechanism of FIG. 7A, the arbitration controlmechanism of FIG. 7A is similar to the mechanism of FIG. 5A, except thatthe mechanism of FIG. 7B includes two stage 3 arbitrations that controlaccess to two banks of cache, namely BANK0 and BANK1. More specifically,stage 3 of FIG. 7B arbitration mechanism includes a two way arbiter(ARB2) 527-1 and a two way arbiter (ARB2) 527-1 that respectively feedarbitration results to sequencer 5128-0 and sequencer 528-1. Thus, stage3 includes two parallel arbiters, namely arbiters 527-0 and 527-1, eachhaving a dedicated sequencer, namely sequencers 528-0 and 528-1,respectively. Sequencers 528-0 and 528-1 each supply load and storerequests to L2 cache BANK0 and L2 cache BANK1, as shown.

Again comparing the arbitration control mechanism of FIG. 7A with thatof FIG. 5A, the stage 3 arbitration mechanism of FIG. 7A replicates thestage 3 arbitration mechanism as two cache banks (BANK0 and BANK1), twosequencers 528-0 and 528-1, and two ARB2 arbiters 527-0 and 527-1, asshown. This increases the effective bandwidth of the L2 cache memory390′. The arbitration mechanism of FIG. 7A provides for the expeditedhandling of load operations to the L2 cache. The arbitration mechanismof FIG. 7A provides a single dispatch point, namely arbiter ARB 526, inthe second stage to feed the load operation and store operation dataflow into the dual cache banks BANK0 and BANK1 via stage 3. As seen inFIG. 7A, the stage 1 arbiter 541 for load operation includes a directpath to both stage 3 arbiters 527-0 and 527-1. In this manner, the loadoperation that arbiter 541 selects in stage 1 may effectively bypass andinterrupt a store operation that stage 3 sends to dual cache banks BANK0and BANK1 for servicing.

FIG. 7B shows a timing diagram that depicts the operation of the dualbank L2 cache of FIG. 7A and FIG. 3B. While stage 3 includes dualarbiters 527-0 and 527-1, stage 2 includes a single arbiter 526. In thisarrangement, in a particular cycle, the L2 cache system 213 may commencea sequence of loads or a sequence of stores, but both sequences do notstart at the same time, i.e. start during the same cache cycle or beat.For example, as seen in FIG. 7B, the L2 cache system 213 receives a loadrequest and, in response, performs the requested load operation in cacheaccesses RO-A, RO-B, RO-C and RO-D during cycles 3, 4, 5 and 6respectively, as read operations to BANK0. The L2 cache system 213receives a store request and, in response, performs the requested writeoperation during cache accesses WO-A, WO-B, WO-C and WO-D to the otherbank of the L2 cache, namely BANK1. These writes commence in cycle 4which is one cycle after the sequence of reads start in cycle 3 toservice the previous load request. The write operations occur duringcycle 4, 5, 6 and 7, to service the store request.

Following the completion of write operation WO-D at cycle 6, L2 cachesystem 213 receives a load request and, in response, performs therequested load operation in cache accesses R1-A, R1-B, R1-C and R1-Dduring cycles 8, 9, 10 and 11, as read operations to BANK1. Once cycleafter this read sequence begins in BANK1, L2 cache system 213 respondsto a store request and performs the requested store operation in cacheaccesses W1-A, W1-B, W1-C and W1-D during cycles 9, 10, 11 and 12,respectively, as writes to BANK0.

Following the completion of write operation W1-D at cycle 12, L2 cachesystem 213 receives a load request and, in response, performs therequested load operation in cache accesses R2-A, R2-B, R2-C and R2-Dduring cycles 13, 14, 15 and 16, as read operations to BANK0. Once cycleafter this read sequence begins in BANK0, L2 cache system 213 respondsto a store request and performs the requested store operation in cacheaccesses W2-A, W2-B, W2-C and W2-D during cycles 14, 15, 16 and 17,respectively, as writes to BANK1. The performance of load/readoperations and store/write operations thus alternates between BANK0 andBANK1 of L2 cache memory 390′.

FIG. 7C is a high level flowchart that depicts process flow in the thirdembodiment of the disclosed L2 cache management methodology. Theflowchart of FIG. 7C exhibits many similarities to the flowchart of FIG.6B discussed above. One difference in the flowchart of FIG. 7C is thatafter L2 cache management system 213 finds a load request at decisionblock 545 and determines that the L2 cache memory 390′ is not busyservicing a load request at decision block 550, then system 213 begins acache access to service the load request on cache bank load needs, asper block 555′. In other words, cache management system 213 need notaccess both banks to retrieve the cache line, but rather accesses thebank in L2 cache memory 390′ that it needs to access to perform thecache line load. System 213 then continues monitoring for more loadrequests at decision block 545.

System 213 performs a test to determine if a request is a store requestat decision block 560. If the request is a store request, then system213 determines the size of the store request, i.e. the number of cyclesor cache bytes that the store request needs to access in the cache linein order to execute the store request, as per block 605, as opposed towriting the entire cache line. After determining the size of a storerequest, cache system 213 determines if the cache is busy on a previousload or store request access that is still yet to complete and is forthe same bank this store request needs, as per decision block 705. Ifcache system 213 finds the cache not to be busy, then system 213 startsa cache access to service the store request. Process flow then continuesin the same manner as the second embodiment of the FIG. 6B flowchart,except that at block 590′ system 213 starts a cache access to service aload request on cache bank load needs.

The method that the FIG. 7C flowchart depicts provides cache access toBANK0 to service a cache read operation at substantially the same timethat it provides access to BANK1 to service a cache write operation.While these read and write operations substantially overlap in time,they do not start on the same L2 cache cycle. There is a one cycle delayfrom the time that one cache bank begins an access in response to arequest to the time that the other cache bank begins an access inresponse to another request. This results is two dead cycles duringwhich a particular cache bank does not service a request, for exampledead cycles 7 and 8 for cache BANK0 and dead cycles 12 and 13 for cacheBANK1 in the FIG. 7B timing diagram.

The fourth embodiment employs the dual bank cache architecture that FIG.3B depicts. FIG. 3B is similar to FIG. 3A except for the dual bank L2cache 390′ architecture that FIG. 3B employs and the arbitration controlmechanism of FIG. 8A. As discussed above, L2 cache memory 390′ includes2 banks of high speed cache memory, namely BANK0 and BANK1, and a singledirectory 491. BANK0 stores even cache lines while BANK1 stores oddcache lines. Multiplexer 395 can select either an even cache line fromBANK0 or an odd cache line from BANK1 of L2 cache memory 390′.

Like the third embodiment, the fourth embodiment discussed below employsdual data interleaving in BANK0 and BANK1 of L2 cache memory 390′.However, in the fourth embodiment, the arbitration mechanism of FIG. 8Amay commence an access to BANK0 for a read operation at the same timethat the arbitration mechanism accesses BANK1 for a write operation, andvice versa. This further increases the bandwidth into and out of the L2cache beyond what the third embodiment provides. In the fourthembodiment, both cache banks may not only simultaneously execute readand write requests respectively, but they may also start executing theread and write requests simultaneously, i.e. in the same L2 cache accesscycle, as seen in the timing diagram of FIG. 8B.

FIG. 8A shows the arbitration control mechanism for the fourthembodiment of the disclosed L2 cache system. Comparing the arbitrationcontrol mechanism of FIG. 8A with the arbitration control mechanism ofFIG. 7A, the arbitration control mechanism of FIG. 8A is similar to themechanism of FIG. 7A, except that the mechanism of FIG. 8A includes twostage 2 arbiters (526-0, 526-1) that control access to the two stage 3arbiters (527-0, 527-1). Replicating the stage 2 arbiter in this mannerenables the stage 2 arbiters to select from all of the state machinesthat contend for the even cache lines of BANK0 and all of the statemachines that content for the odd cache lines of BANK1. Arbiters 526-0and 526-1 may then cause sequencers 528-0 and 528-1 to start L2 cacheaccesses to BANK0 and BANK1 at the same time. This enables fullutilization of cache banks BANK0 and BANK1 without dead cycles. Thearbitration mechanism of FIG. 8A provides dual dispatch points, namelyarbiters 526-0, 526-1 in the second stage to feed the read operation andwrite operation data flow into the dual cache banks BANK0 and BANK1 viastage 3.

FIG. 8B shows the timing diagram for the fourth embodiment of the L2cache system that depicts the operation of the dual bank L2 cache ofFIG. 8A and FIG. 3B. In this embodiment, not only stage 3 includes dualarbiters, but also stage 2 includes dual arbiters 526-0 and 526-1. Inthis arrangement, in a particular cycle, the L2 cache system 213 maycommence a sequence of reads or a sequence of writes, and both sequencesmay start accesses to the respective cache banks at the same time, i.e.start during the same cycle or beat. For example, as seen in FIG. 8B,the L2 cache system 213 receives a load request and, in response,performs the requested load operation in cache accesses RO-A, RO-B, RO-Cand RO-D during cycles 3, 4, 5 and 6 respectively, as read operations toBANK0. The L2 cache system 213 receives a store request and, inresponse, performs the requested write operation during cache accessesWO-A, WO-B, WO-C and WO-D to the other bank of the L2 cache, namelyBANK1. Both the read sequence and the write sequence begin in the samecycle 3. The write operations occur during cycle 3, 4, 5 and 6, toservice the store request. These are the same cycles that cache system213 employs to service the load request.

Subsequent cache accesses to service cache read and write requests maythen begin in cycle 7 without any dead cycles between ending a cacheread access and starting a cache write access, and vice versa. Forexample, L2 cache system 213 receives a cache write request and, inresponse, commences cache accesses W1-A, W1-B, W1-C and W1-D duringcycles 7, 8, 9 and 10. L2 cache system 213 receives a cache read requestand, in response, commences cache accesses R1-A, R1-B, R1-C and R1-Dduring the same cycles 7, 8, 9 and 10 that system 213 employs to servicethe cache write request.

Subsequent cache accesses to service store and cache read and writerequests may then begin in cycle 11 without any dead cycles betweenending a cache read access and starting a cache write access, and viceversa. For example, L2 cache system 213 receives a load request and, inresponse, commences cache accesses R2-A, R2-B, R2-C and R2-D duringcycles 11, 12, 13 and 14. L2 cache system 213 receives a cache writerequest and, in response, commences cache accesses W2-A, W2-B, W2-C andW2-D during the same cycles 11, 12, 13 and 14 that system 213 employs toservice the cache store request.

FIGS. 8C and 8D together form a high level flowchart that depictsprocess flow in the fourth embodiment of the disclosed L2 cachemanagement methodology. The flowchart of FIGS. 8C and 8D exhibits manysimilarities to the flowchart of FIG. 6B discussed above, except for thedifferences discussed below. These differences involve L2 cache system213 starting an access of one cache bank to service a load request andat the same time starting an access of the other cache bank to service astore request, and vice versa.

As in the third embodiment of FIG. 7C., the fourth embodiment monitorsfor load requests at decision block 545 and determines if the cachebanks are currently busy servicing a load request at decision block 550.After receiving a load request and determining that the cache memory390′ is not busy servicing another request, system 213 performs a testto determine if it also received a store request that desires access toone of the cache banks, as per decision block 805. If system 213 didreceive such a store request, then system 213 performs an additionaltest to determine if the received load request and the received storerequest are for different cache banks, as per decision block 810. If theload request and the store request are not for different L2 cache banks,then arbitration mechanism of FIG. 8B allows the load request to winover the store request. In this event, L2 cache system 213 starts acache access to service the load request based on the actual size needsof the load request, as per block 555′. However, if the load request andthe store request are to different cache banks, then the arbitrationmechanism sets up respective cache accesses to service the load andstore requests. More particularly, L2 cache system 213 determines thesize of the store request, namely the number of cache cycles or cachebytes needed to execute the store request, as per block 815. System 213starts a cache access on one of the dual cache banks BANK0 and BANK1 toservice the load request and starts an access of the remaining cachebank to service the store request, both accesses starting during thesame cache cycle. Process flow then continues to decision block 540 asbefore.

Another difference in the flow chart of FIGS. 8C and 8D is that after L2cache system 213 receives a load request, as per decision block 575, anddetermines that the cache memory is not already busy servicing anotherload request, as per decision block 580, system 213 performs anothertest to determine if the cache is busy servicing bank load requestneeds, as per decision block 825. In other words, decision block 825tests to determine if the L2 cache memory 390′ is currently busingservicing a load request according to the size or time requirements thatthe particular load request actually needs. If the L2 cache memory isbusy servicing a load request according to its size or time needs indecision block 825, then system 213 interrupts a store request that isin progress accessing the cache memory to service the interrupting loadrequest. However, if decision block 825 determines that the cache memory390′ is not currently busy servicing bank load request needs, then cachesystem 213 proceeds with servicing the current store cache accesses orrestarting the interrupted store cache access and also starting a loadcache access, as per block 830. This load cache access is to a differentcache bank than the current store cache access or the restartedinterrupted store cache access. Process flow then continues back todecision block 570′ at which system 213 tests to determine if system 213completed a store-sized access resulting from a load request, as before.

In summary, the L2 cache system 213 of the fourth embodiment employsdual second stage arbiters, dual third stage arbiters and dual cachebanks BANK0 and BANK1 to enable the system to service a load request anda store request beginning at the same time without the occurrence ofefficiency degrading dead cycles. System 213 may assign a load requestto one cache bank while assigning a store request to the other cachebank, and vice versa. The resultant load operation may occur atsubstantially the same time as the resultant store operation such thatthe load operation and the store operation overlap in time. Thearbitration mechanism provides that a load operation may interrupt astore operation already in progress in a particular cache bank.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A method comprising: sending, by a processor element, a plurality of requests for memory operations to a cache memory, the cache memory including first and second cache banks, the memory operations including load operations and store operations, each load and store operation exhibiting a respective size requirement; arbitrating, by an arbitration mechanism, among the plurality of requests for memory operations to select a particular load operation and a particular store operation for access to the cache memory; commencing, by the arbitration mechanism, the particular load operation on the first cache bank during a first cache cycle; commencing, by the arbitration mechanism, the particular store operation on the second cache bank during a second cache cycle that follows the first cache cycle; and performing, by the first and second cache banks, the particular load operation and the particular store operation substantially simultaneously.
 2. The method of claim 1, further comprising interrupting with a load operation, by control logic, a particular store operation that is in progress in one of the first and second cache banks.
 3. The method of claim 1, further comprising: determining, by control logic, the respective size requirements of the store operations and the load operations in the plurality of requests; performing a first cache memory access, by the cache memory, in response to a load operation request, the first cache memory access being limited in size to the size requirement for the load operation determined by the control logic; and performing a second cache memory access, by the cache memory, in response to a store operation request, the second cache memory access being limited in size to the size requirement for the store operation determined by the control logic.
 4. The method of claim 1, further comprising: arbitrating, by the arbitration mechanism, in a first arbitration stage among the load operation requests in the plurality of requests for memory operations, to provide the particular load operation for access to the cache memory; arbitrating, by the arbitration mechanism, in the first arbitration stage among store requests, to provide the particular store operation for access to the cache memory; and arbitrating, by the arbitration mechanism, in the first arbitration stage among read claim state machine requests, cast out state machine requests and snoop requests to determine a cache arbiter arbitration result.
 5. The method of claim 4, further comprising: arbitrating, by the arbitration mechanism, in a second arbitration stage between the selected store operation and the cache arbiter arbitration result to select a second stage arbitration result.
 6. The method of claim 5, wherein the second stage arbitration result is the particular store operation, the method further comprising arbitrating, by the arbitration mechanism, in a third arbitration stage that includes first and second arbiters that operate in parallel to provide the particular load operation to the first cache bank and to provide the particular store operation to the second bank for substantially simultaneous access to the cache memory.
 7. The method of claim 1, wherein the cache memory is an L2 cache memory.
 8. The method of claim 1, wherein the processor element is a processor core that includes an L1 cache memory.
 9. A cache memory system comprising: a processor element; a cache memory, coupled to the processor element, that receives a plurality of requests for memory operations, the cache memory including first and second cache banks, the memory operations including load operations and store operations, each load and store operation exhibiting a respective size requirement, the cache memory including an arbitration mechanism that arbitrates among the plurality of requests for memory operations to select a particular load operation and a particular store operation for access to the cache memory, wherein the arbitration mechanism commences the particular load operation on the first cache bank during a first cache cycle and commences the particular store operation on the second cache bank during a second cache cycle that follows the first cache cycle, the first and second cache banks performing the particular load operation and the particular store operation substantially simultaneously.
 10. The cache memory system of claim 9, wherein the cache memory includes control logic that interrupts, with a load operation, a particular store operation that is in progress in one of the first and second cache banks.
 11. The cache memory system of claim 9, wherein the control logic determines the respective size requirements of the store operations and the load operations in the plurality of requests, wherein the cache memory performs a first cache memory access in response to a load operation request, the first cache memory access being limited in size to the size requirement for the load operation determined by the control logic, wherein the cache memory performs a second cache memory access in response to a store operation request, the second cache memory access being limited in size to the size requirement for the store operation determined by the control logic.
 12. The cache memory system of claim 9, wherein the arbitration mechanism comprises: a first arbitration stage that arbitrates among the load operation requests in the plurality of requests for memory operations, to provide the particular load operation for access to the cache memory, the first arbitration stage arbitrating among store requests to provide a selected store operation for access to the cache memory, the first arbitration stage arbitrating among read claim state machine requests, cast out state machine requests and snoop requests to determine a cache arbiter arbitration result.
 13. The cache memory system of claim 12, wherein the arbitration mechanism further comprises: a second arbitration stage, coupled to the first arbitration stage, that arbitrates between the selected store operation and the cache arbiter arbitration result to select a second stage arbitration result.
 14. The cache memory system of claim 13, wherein the second stage arbitration result is the particular store operation, the arbitration mechanism further comprising: a third arbitration stage, coupled to the second arbitration stage, that arbitrates to provide the particular load operation to the first cache bank and to provide the particular store operation to the second bank for substantially simultaneous access to the cache memory.
 15. The cache memory system of claim 9, wherein the cache memory is an L2 cache memory.
 16. A information handling system (IHS) comprising: a processor element; a cache memory, coupled to the processor element, that receives a plurality of requests for memory operations, the cache memory including first and second cache banks, the operations including load operations and store operations, each load and store operation exhibiting a respective size requirement, the cache memory including an arbitration mechanism that arbitrates among the plurality of requests for memory operations to select a particular load operation and a particular store operation for access to the cache memory, wherein the arbitration mechanism commences the particular load operation on the first cache bank during a first cache cycle and commences the particular store operation on the second cache bank during a second cache cycle that follows the first cache cycle, the first and second cache banks performing the particular load operation and the particular store operation substantially simultaneously; and a system memory coupled to the cache memory.
 17. The IHS of claim 16, wherein the arbitration mechanism comprises: a first arbitration stage that arbitrates among the load operation requests in the plurality of requests for memory operations, to provide the particular load operation for access to the cache memory, the first arbitration stage arbitrating among store requests to provide a selected store operation for access to the cache memory, the first arbitration stage arbitrating among read claim state machine requests, cast out state machine requests and snoop requests to determine a cache arbiter arbitration result.
 18. The IHS of claim 17, wherein the arbitration mechanism further comprises: a second arbitration stage, coupled to the first arbitration stage, that arbitrates between the selected store operation and the cache arbiter arbitration result to select a second stage arbitration result.
 19. The IHS of claim 18, wherein the second stage arbitration result is the particular store operation, the arbitration mechanism further comprising: a third arbitration stage, coupled to the second arbitration stage, that arbitrates to provide the particular load operation to the first cache bank and to provide the particular store operation to the second bank for substantially simultaneous access to the cache memory.
 20. The method of claim 1, wherein the cache memory is an L2 cache memory. 